Production, scavenging and toxicity of hydrogen peroxide in the green seaweedUlva rigida

Some aspects of hydrogen peroxide (H2O2) production, scavenging and toxicity in Ulva rigida (Chlorophyta) have been investigated. The seaweed produced H2O2 in light and excretion increased exponentially with increasing light up to 800 μmol photons m-2 s-1 where the excretion rate was 330 μmol (kg Chl a)-1s-1. This corresponded to a concentration of 4 μMH2O2 in the experimental medium after 30 min (density of seaweed: 9·2 kg fresh weight m-3). At 1200 μmol photons m-2 s-1, the excretion rate decreased to 190 μmol (kg Chl a)-1 s-1. At 600 μmol photons m-2 s-1, the excretion caused a concentration of 1·8 μM H2O2 in the experimental seawater; after a further 2 h the concentration decreased to a steady level of c. 0·8 μM. The enzymatic degradation of H2O2 is suggested to be primarily through the activity of ascorbate peroxidase (62 mmol H2O2 (kg Chl a)-1 s-1) since no catalase activity was detected. The H2O2 levels in U. rigida are reduced by diffusion through the plasmalemma. Addition of 1 mM H2O2 caused an 8...